Te Mihi Geothermal Power Station, New Zealand

The 166MW Te Mihi geothermal power plant is being constructed by Contact Energy on the Wairakei geothermal steamfield in Taupo, New Zealand.

Construction of the $623m plant is currently underway and is expected to be completed by mid-2013.Upon completion, 45MW of the existing 52-year-old Wairakei geothermal power plant will be decommissioned.

Combined output from the two stations will be 114MW, which will be enough to power more than 110,000 homes.The Wairakei power plant has been subject to frequent maintenance and refurbishments and is expected to reach the end of its useful life by 2026.

The remaining units of the power station will be decommissioned in 2026. A third unit at the Te Mihi station may be added at this time.The Te Mihi project is part of a $750m programme to develop the Wairakei-Tauhara geothermal system.

Power generated by the new plant will help in fostering economic growth in the region and providing a secure source of electricity supply.

Development of the Te Mihi geothermal power plant

“The 166MW Te Mihi geothermal power plant is being constructed by Contact Energy.”

The Wairakei power plant was earlier supplied with steam from the eastern and western borefields on the Wairakei geothermal reservoir. The steam produced by this borefield, however, has been on the decline.

The Te Mihi borefield situated 5km from the Wairakei power plant has, therefore, become the main source of steam supply to the plant.

As the production wells are located farther away from the existing power plant, energy loss from steam transmission has increased.

Building a new plant near the Te Mihi borefield was considered as the best solution to ensure optimal usage of the available geothermal fluid.

Construction of the new plant is also expected to bring environmental benefits. The water to cool the Wairakei power plant is currently being sourced from the Waikato River.

The used water containing small amounts of hydrogen sulphide is discharged back into the river, polluting the river water.

To avoid this pollution, a biological treatment facility will be built to remove hydrogen sulphide. Wells and piping systems are also included in the project, which will enable the separated geothermal water to be reinjected instead of discharging into the river.

Te Mihi Geothermal Power Station details

The Te Mihi power plant will feature two 83MW steam turbine generators using dual-flash steam separation.

“Both the IP steam and LP steam are passed to the Te Mihi steam turbine generator sets to produce electricity.”

Infrastructure at the plant will include a turbine hall, cooling towers, gas extraction equipment, steam discharge silencers and condensate pipelines.

New production and reinjection wells will be drilled into the Te Mihi borefield which generates a mix of dry steam and two phase fluid. The new plant will also draw two phase geothermal fluid from the western borefield.

Fluid and steam drawn from the wells is separated into intermediate pressure (IP) steam and geothermal water. The water is flashed to produce low pressure (LP) steam.

Both the IP steam and LP steam are passed to the Te Mihi steam turbine generator sets to produce electricity.

The condensed geothermal fluid and steam condensate are injected back into the ground. Non-condensable gases are removed by gas extractors and discharged to the atmosphere by mixing into the thermally buoyant cooling tower discharge plume.

Wairakei-Tauhara geothermal system grid network

The project includes construction of a switchyard and 2km of 220kV transmission line.This new line will be connected to the 220kV Wairakei – Whakamaru transmission line which is part of the national grid. The current Poihipi Road power station spur line will also be connected to the new Te Mihi switchyard.

Key players in plant construction

A consortium consisting of McConnell Dowell, SNC-Lavalin and Parsons Brinckerhoff has been awarded the engineering, procurement and construction contract for the plant.

Toshiba International Corporation, a subsidiary of Toshiba Corporation, won a contract to provide steam turbine generator sets for the plant. The turbines are scheduled for delivery in 2012.

MB Century is responsible for the detailed design of the steamfield development which will supply steam to the power plant.